This invention relates to an electroacoustical device and, more particularly, to a loudspeaker for reproduction of low audible frequencies. Specifically, this invention relates to a small dimension low frequency folded exponential horn loudspeaker which has a unitary sound path for direction of acoustical waves from an electroacoustic transducer to a volume into which the acoustical waves are radiated. Moreover, this invention relates to a loudspeaker system, including a low frequency loudspeaker and midrange and high frequency loudspeakers and an LC crossover network, which operates at optimum efficiency and which has a smooth amplitude response over the range of audible frequencies that is necessary for high fidelity sound reproduction.
High fidelity sound reproduction requires reproduction of low audible frequencies. W. B. Snow, "Audible Frequency Ranges of Music, Speech, and Noise," Jour. Acous. Soc. Am., Vol. 3, July, 1931, p.155, for example, indicates that high fidelity sound reproduction of orchestral music requires that the frequency band should extend to as low as 40 Hz.
It is well established that loudspeakers, in order to reproduce a given frequency range, must have dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn loudspeaker, for example, the area of the exponential horn mouth is determined on the basis of the wavelength of the lowest frequency to be reproduced.
At an early date, to obtain high fidelity sound reproduction with exponential horn loudspeakers, and, in particular, the inclusion of low audible frequencies, large exponential horn loudspeakers were constructed. For example, theater loudspeakers as large or larger than eight feet in length and four feet by four feet in transverse dimensions were built in order to obtain reproduction of low audible frequencies. Later, the outside dimensions of the exponential horns were reduced by folding, but even then the dimensions of the mouths were large for reproduction of low audible frequencies. More recently, folded exponential horn loudspeakers with reduced mouth dimensions have been used in proximity to boundary surfaces, such as a floor, a ceiling, and/or walls of a room, to increase the effective mouth area so that low audible frequencies are reproduced while at the same time the dimensions of the low frequency loudspeakers are minimized. See, for example, Sandeman, U.S. Pat. No. 1,984,550, Klipsch, U.S. Pat. Nos. 2,310,243 and 2,373,692, and Klipsch, "La Scala," Audio Engineering Society Preprint No. 372, April, 1965.
Prior art low frequency folded exponential horn loudspeakers, such as those which are disclosed in the above-cited references, have small dimensions and, when their mouths are located proximate planar surfaces, enable reproduction of low audible frequencies. However, each of these prior art low frequency folded exponential horn loudspeakers is structurally complex due to the structure of the folded exponential horn which defines the sound path from the electroacoustic transducer to the volume into which sound is radiated. Perhaps the simplest construction appears in the above-cited Audio Engineering Society publication. In that construction, the folded exponential horn is bifurcated to define a double sound path.
Due to the complex structure, the production of high fidelity, small dimension, low frequency folded exponential horn loudspeakers has required considerable craftsmanship. High quality control in manufacture has been necessary to assure that the construction meets specifications. Consequently, the cost of low frequency folded exponential horn loudspeakers has been high. Furthermore, the amount of material which has been used in some structurally complex prior art low frequency folded exponential horn loudspeakers has meant that these loudspeakers do not have the high degree of portability which is required by traveling musicians.
One objective of this invention is to provide a low frequency loudspeaker of the folded exponential horn type which cooperates with boundary surfaces so that reproduction of low audible frequencies is obtained with a loudspeaker that has small dimensions in relation to the wavelength of the lowest audible frequency to be reproduced.
Another objective of this invention is to provide a low frequency folded exponential horn loudspeaker with a simplified structure without sacrificing high fidelity sound reproduction or small dimensions.
Another objective of this invention is to provide a comparatively low cost folded exponential horn loudspeaker.
Another objective of this invention is to provide a low frequency loudspeaker for the traveling musician who needs highly portable sound reinforcement over the low vocal range and the bass range of some musical instruments.
A further objective of this invention is to provide a loudspeaker system, including a low frequency folded exponential horn loudspeaker and midrange and high frequency loudspeakers and an LC crossover network, for reproduction of audible frequencies of the acoustical spectrum without harmonic distortion.
Another objective of this invention is to provide a loudspeaker system which operates at optimum efficiency.
Another objective of this invention is to provide a loudspeaker system which has a smooth amplitude response over the range of audible frequencies that is necessary for high fidelity sound reproduction.
A further objective of this invention is to enhance the overall smoothness of the amplitude response of a loudspeaker system over a range of audible frequencies.
An additional objective of the present invention is to provide a loudspeaker system which can be used for radiation of sound into either a .pi. solid angle or a .pi./2 solid angle without deterioration of smoothness of amplitude response.